As is well known, a 9% nickel steel is a high tensile strength steel to be used at cryogenic temperatures to −196° C. or less, and has a high proof stress and an excellent low-temperature toughness. Accordingly, the 9% nickel steel has been widely used for a storage tank of LNG, liquid nitrogen, liquid oxygen, or the like, related equipment thereof, or the like. Thus, the 9% nickel steel has an excellent cryogenic toughness. However, in order to utilize this feature, as a matter of course, the welded joint part is also required to have equivalent cryogenic characteristics.
From such background, up to now, various studies have been also made on a welding technology of a cryogenic steel, leaving many insufficient aspects from the standpoint of satisfying both the cost effectiveness and the cryogenic characteristics. For example, it can be considered as follows: when a cryogenic steel is welded using a welding wire having components similar to those of a cryogenic steel (so-called similar composition metal type wire), a welded joint excellent in cryogenic characteristic can be obtained. However, with the current welding method, a stable low-temperature toughness cannot be ensured in the as-welded state. Further, for a welded structure of a cryogenic steel, a heat treatment for recovering the toughness after completion of welding is very difficult. Accordingly, a welding wire having components similar to those of the cryogenic steel is not practical.
For this reason, for welding of a cryogenic steel, mainly, a high nickel alloy welding wire has been often used. However, a welded joint using a high nickel alloy welding wire shows an excellent toughness at −196° C. even in the as-welded state, but is much lower in tensile strength, particularly, 0.2% proof stress, than a 9% nickel steel (base metal). As a result, despite use of a 9% nickel steel as a 70-kg/mm2 class high tensile strength steel, the strength of the welded joint part is low. This also forces the design stress to be accordingly reduced. In order to ensure the strength, disadvantageously, the plate thickness of the whole welded structure must be increased.
Therefore, so long as a high nickel alloy welding wire is used, the high strength of a 9% nickel steel cannot be utilized sufficiently. This imposes double economic burdens of an increase in plate thickness of the welded structure and an increase in quantity of an expensive high nickel alloy welding wire consumed. Further, with welding by a high nickel alloy, a problem of high temperature cracking inherently occurs. In addition, a large difference in components from a 9% nickel steel, which is the base metal, also creates a problem of thermal fatigue due to a difference in thermal expansion coefficient, or the like.
From the foregoing reasons, a 9% nickel steel has excellent performances as a cryogenic steel. In spite of this, in actuality, the scope of application thereof is remarkably limited.
Regarding the welding technology using a similar composition metal type welding wire similar in components to a 9% nickel steel base metal, there has been conventionally conducted a study in order to enhance the cryogenic characteristics of a welded joint part. For example, in JP-A No. 54-76452, or the like, there is disclosed a method for improving the foregoing problems by focusing attention on the chemical components of the similar composition metal type welding wire, particularly by adjusting and limiting the contents of nickel, manganese, boron, oxygen, and the like in the welding wire within the proper ranges. However, with this method, although the low-temperature toughness improvement results of the welded joint part by the Charpy impact test according to JIS-Z-3111 has been reported, the results thereof are based on evaluation by only the whole absorption energy. Thus, no effort has been made from a viewpoint of crack initiation necessary for ensuring the safety as an actual large-size welded structure. Therefore, with this method, in evaluation by only the absorption energy, a sufficient low-temperature toughness satisfying the criterion thereof is obtained. However, there has still been room for improvement on the crack initiation resistance (crack inhibiting strength) reflecting even the real crack initiation as further described later.
Whereas, in JP-A No. 53-118241 or the like, there is proposed a method in which the low-temperature toughness of the welded joint part is improved by devising how to execute welding. Namely, in this publication, there is disclosed the following method: after performing multilayer welding, the weld bead surface of the final layer is cooled to 150° C. or less, and then, the weld bead surface of the final layer is remolten with an arc from a non-consumable electrode while being shielded by an inert gas. This method is intended to achieve the following: the central part (lower layer part) of the groove can receive a proper heat treatment effect by the heat cycle during upper layer part welding, and hence the low-temperature toughness of the lower layer part is enhanced; however, the final layer cannot expect to receive the heat treatment effect; for this reason, the final layer is remolten, thereby to be applied with a heat treatment to improve the low-temperature toughness. However, this method has a problem of an increase in number of steps in executing welding. This method is limited to the improvement of the partial low-temperature toughness of only the final welded layer in the welded joint part to the end. Therefore, unfavorably, this method inherently has a problem of having a limit in improving the low-temperature toughness of the whole weld metal controlling the characteristics of the welded joint. Further, also with this method, as with the prior art technology, there have been confirmed only the low-temperature toughness improving effect by a simple Charpy impact test or COD test. There has still been room for efforts from a viewpoint of crack initiation necessary for ensuring the safety as an actual large-size welded structure.
Whereas, regarding the improvement of the low-temperature toughness of a nickel-containing steel, there is proposed a technology of shortening the time of the heat treatment of the similar composition metal weld part of the nickel-containing steel in JP-A No. 61-15925. In this publication, the low-temperature toughness is ensured by control of the carbide form and the heat treatment after welding. In this case, although the reason for addition thereof is not clear, a wire obtained by adding a REM in an amount of 0.042% or more is used in Examples. Also with this technology, as with the JP-A No. 53-118241, a heat treatment after welding is required. This entails an increase in number of steps, and further an increase in cost. Further, the study on the wire components is insufficient. Therefore, as with the prior-art technology, also with this method, there has still been room for efforts from a viewpoint of crack initiation necessary for ensuring the safety as an actual large-size welded structure.